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Old 08-02-2014, 11:46 AM   #29
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Originally Posted by John Hilley View Post
This doesn't make sense either. If the CPAP takes 1 amp on 120 VAC, it should take close to 10 amps on 12 VDC. There could be a little lost with the 120 VAC conversion, but not much.
True story: I used to think MSW inverters were more efficient than True Sine till I researched.. So what did I find.

I looked at TWO Xantrex converters. One a Prosine (true sine) and one a X-power (MSW) as it happens I have one of each here but NOT the specific ones I looked at.

I made sure they were the same size (Apples to as they say)

The X-power had a peak efficiency of 90%
The Prosine 89-90 percent

So based on that a 120 volt device drawing 1 amp will need a MINIMUM (Not both cases it was BEST) of 11 amps at 12 volt to feed the inverter

This is a guess. But I am guessing peak efficiency happens close to maximum load. (That would be 1500 watts on the two I compared)

I normally say that the 12 volt amp draw is = to the 120 volts WATTS/10

Now I know that is not accurate.. but as it turns out it is very close for anything under 1,000 watts.

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Old 08-04-2014, 08:13 AM   #30
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The only problem in your calculation is the inverter has an over head running but doing nothing. A few amps at idle, maybe 2-6 amps so at a draw of 10 amp load, a very low load, May cost you 16 amps.


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